Isotope hydrology and hydrogeochemical signatures in the Lake Malawi Basin: a multi-tracer approach for groundwater resource conceptualisation

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Abstract

Reliance on groundwater is outpacing natural replenishment, a growing imbalance that requires detailed and multi-faceted water resource understanding. This study integrated water-stable isotopes and hydrogeochemical species to examine hydrogeochemical processes during groundwater recharge and evolution in the Lake Malawi basin aquifer systems. The findings provide insights into groundwater source provenance, with non-evaporated modern precipitation dominating recharge inputs. Grouped hydrochemical facies exhibit five groundwater water types, prominently featuring Ca-Mg-HCO3. Modelled hydrogeochemical data underscore dominant silicate dissolution reactions with the likely precipitation of calcite and/or high-Mg dolomitic carbonate constrained by ion exchange. Isotope hydrology reinforces water resource system conceptualisation. Coupled isotopic-hydrogeochemical lines of evidence reveal a discernible spatial-seasonal inhomogeneity in groundwater chemical character, revealing a complex interplay of meteoric water input, evaporative effects, recharge processes, and mixing dynamics. Findings show that measurable nitrate across Malawi highlights a widespread human impact on groundwater quality and an urgent need for detailed modelling to predict future trends of nitrate in groundwater with respect to extensive fertiliser use and an ever-increasing number of pit latrines and septic systems arising from rapid population growth. This study not only refined the Lake Malawi basin aquifer systems conceptualisation but also provided isotopic evidence of groundwater and lake water mixing. This study sets a base for groundwater management and policy decisions in support of the Integrated Water Resources Management principles and Sustainable Development Goal 6 objectives for groundwater sustainability in the transboundary Lake Malawi basin.
Original languageEnglish
Article number1587
Number of pages27
JournalWater
Volume16
Issue number11
DOIs
Publication statusPublished - 31 May 2024

Funding

The authors are thankful for the financial support of the Scottish Government under the Climate Justice Fund Water Futures Program (research grant HN-CJF-03), awarded to the University of Strathclyde (Prof. R.M. Kalin PI). We also gratefully acknowledge the funding by the Commonwealth Scholarship Commission (CSC) awarded to Limbikani Chitsundi Banda, the principal researcher. Finally, we acknowledge the International Atomic Energy Agency (IAEA) research grant awarded to Limbikani Chitsundi Banda (Prof R.M. Kalin Technical Advisor) under the AFRA Regional Project grant (RAF7021). We acknowledge the administrative support by the Malawi Government through the Ministry of Water and Sanitation and the Civil and Environmental Engineering Department of the University of Strathclyde. The overarching aim of this work was to establish a conceptualisation of the groundwater in the Lake Malawi basin (LMB) aquifer systems, focusing on isotope hydrology and hydrogeochemical baseline characterisation, spatial-seasonal effects on groundwater mineral signatures, geochemical controls on groundwater quality and evolution, groundwater-resource provenance, and hydrogeological system conceptualisation. Specifically, this study addressed the following research questions: (a) What are the background isotope hydrology and hydrochemical signatures of groundwater in the LMB? (b) How are the LMB aquifer systems recharged, and what are the isotope hydrology and hydrochemical signatures of distinct recharge inputs? (c) What are the geospatial and seasonal effects on groundwater quality and mineralisation? (d) What are the key geochemical controls on groundwater quality and evolution? and (e) How do isotope hydrology, hydrogeochemical and geospatial characteristics help refine the conceptualisation of the LMB aquifer systems? This study is consistent with the IWRM principles and SDG 6 objectives, with the potential to underpin sustainable groundwater resource management in Malawi and the wider region. This study aligns with the International Atomic Energy Agency (IAEA) Technical Cooperation (TC) project objectives (RAF7021 regional project) for sustainable water management and supports transboundary water management initiatives under the 2004 ZAMCOM agreement and was supported by the Scottish Government\u2019s Climate Justice Fund Water Futures Programme (CJF).

Keywords

  • isotope tracers
  • hydrogeochemistry
  • sustainable development goal 6 (SDG 6)
  • geochemical modelling
  • groundwater sustainability

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